The present invention relates generally to the field of cardiac therapy devices, and particularly to a tachyarrhythmia detection (diagnostic) algorithm.
Implantable cardioverter defibrillators (ICDs) are highly sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient's heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle.
Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart beat. Cardiac arrhythmias are broadly divided into two major categories, namely, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart rate (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart rate (e.g., less than 50 beats/minute). A normal cardiac rhythm (e.g., between 50-100 beats/minute) is referred to as a "sinus rhythm".
Tachyarrhythmias arc further subdivided into two major sub-categories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized.
Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular. In general, atrial arrhythmias are not life-threatening, because the atria (upper chambers of the heart) are only responsible for aiding the movement of blood into the ventricles (lower chambers of the heart), whereas ventricular arrhythmias are life-threatening, because if the ventricles become arrhythmic, the heart's ability to pump blood to the rest of the body is impaired. The most serious and immediately life-threatening type of cardiac arrhythmia is ventricular fibrillation, in which the electrical activity of the ventricles becomes so random and chaotic that the heart rapidly becomes unable to pump sufficient blood to sustain life.
In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrical signals, known as electrograms (EGMs), generated by sensing electrodes positioned proximate to the sino-atrial and/or atrio-ventricular node of the patient's heart, and, most advantageously, in the right ventricular apex of the patient's heart. More particularly, contemporary ICDs include waveform digitization circuitry which digitizes the analog EGM produced by the sensing electrodes, and a microprocessor and associated peripheral ICs which analyze the thusly digitized EGM in accordance with a diagnostic or detection algorithm implemented by software stored in the microprocessor. Contemporary ICDs are generally capable of diagnosing (detecting) the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical energy/therapy to the patient's heart, in accordance with a therapy delivery algorithm also implemented in software stored in the microprocessor, to thereby convert or terminate the diagnosed arrhythmia.
In such ICDs, it is imperative that the detection or diagnostic algorithm employed be reliably accurate, so that the patient's heart condition can be accurately monitored at all times and any arrhythmias promptly and properly diagnosed and treated (by delivery of the appropriate therapy to terminate or convert the detected arrhythmia). In this regard, there are a number of presently available or known detection algorithms which, for the most part, are quite reliable and accurate.
For example, U.S. Pat. No. 4,971,058, issued to Pless et al. and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference, discloses a detection algorithm (hereinafter referred to as "the '058 detection algorithm") which determines the duration of the intervals between successive heartbeats (i.e., cycle lengths between consecutive QRS complexes), and computes a running average of the duration of a prescribed number (e.g., 4) of preceding intervals, referred to as the "average interval", which is re-computed (updated) every interval (i.e., on an interval-by-interval basis). These computed average intervals are used in providing a rhythm diagnosis.
Obviously, a misdiagnosis of sinus tachycardia such as results from exercise as ventricular tachycardia would result in a subsequent delivery of therapy (treatment) which is inappropriate. If therapy is delivered that is not required it may actually induce an arrhythmia that really does require treatment, and, at a minimum, will result in a waste of the finite amount of energy that the device is capable of delivering over its lifetime, thereby shortening its useful lifetime.
One algorithm which is know for diagnosing cardiac rhythms and particularly ventricular tachycardia is called rate stability or interval irregularity. The basic idea is that the interbeat intervals in a normal cardiac rhythm are not regular, i.e., they typically vary from one to the next. However, the interbeat intervals for certain arrhythmias and in particular intervals for successive ventricular tachycardia rhythms tend to be quite regular or stable. It is thus known to look at the variation from one interval to the next to diagnose ventricular tachycardia. Such a diagnostic algorithm is disclosed in U.S. Pat. No. 4,830,006 to Haluska et al, which patent is incorporated herein by reference. An interval irregularity measure may also be used to distinguish atrial fibrillation, which may produce greater interval irregularity, from monomorphic VT which is typically regular.
One problem with prior rate stability algorithms is that they are more computationally demanding than is desirable in an implanted device having a finite energy supply from its battery. Further, some prior art algorithms may be sensitive to cardiac sensing errors.
It is therefore an object of the invention to provide an improved interval irregularity diagnostic algorithm.
It is another object of the invention to provide an interval irregularity algorithm which is less sensitive to cardiac sensing errors.
It is still another object of the invention to provide an interval irregularity algorithm which is not computationally demanding.